The present invention relates to stereophonic broadcasting, and more particularly to a compatible AM stereo system employing a modified quadrature modulation scheme.
The broadcasting community has long sought an acceptable system for transmitting and receiving stereophonic signals in the AM frequency band. To be acceptable, any proposed system must be capable of providing good stereo performance while still being compatible for use with currently available (monophonic) AM receivers. This problem has undergone extensive investigation and many systems have been proposed which meet the above criterion with greater or lesser degrees of success. Since each of these systems has also included undesirable features, however, none has yet met with general acceptance.
Preferably, the stereophonic performance of an acceptable AM stereo system should be such that the signal-to-noise ratio is not significantly degraded as compared to that reception obtainable with current systems. Also, distortion introduced by the transmission and reception of the stereo signal should be minimal. Finally, the separation between the left and right channels should be as great as possible.
With respect to mono-compatibility, any acceptable AM stereo system must be fully compatible with receivers currently on the market. More specifically, no noticeable distortion should be introduced through the detection of the stereo signals with the monophonic envelope detectors and product detectors currently in use. Additionally, the loss in the loudness of the received signal in monophonic receivers due to the stereophonic nature of the broadcast signal should be as low as possible.
The present invention provides a compatible AM stereo system which closely meets all of these requirements. There is disclosed herein a modified quadrature modulation scheme wherein the phase angle between the left (L) and right (R) vector components of the composite stereo AM signal is reduced from the standard 90.degree. (.+-.45.degree. with respect to the carrier) employed in quadrature modulation, to a much narrower angle, nominally equal to 30.degree. (.+-.15.degree. with respect to the carrier). As will be made clearer hereinafter, this system could equivalently be viewed as involving the weighting of the in-phase (L+R) and the quadrature phase (L-R) components of a standard quadrature modulated signal by factors of Cos .theta. and Sin .theta., respectively, where .theta. is again nominally equal to 30.degree..
Systems have been proposed in the past wherein quadrature modulation schemes were utilized specifying modulation angles of less than 90.degree.. The patent to Collins No. 3,231,672, for example, specifies a modulation angle within the range of 55.degree. and 75.degree., whereas the patent to Barton Pat. No. 3,102,167 specifies an angle within the range of 50.degree. to 60.degree.. It has been found in practice that the performance provided by systems employing angles within these ranges is unacceptable. Rather, a compatible AM stereo system may only be provided if modulation angles within the range of 20.degree. to 40.degree., preferably 30.degree. (.+-.15.degree. with respect to the carrier) are employed. When modulation angles within this range are utilized in the schemes set forth in this disclosure, it has been found that excellent mono-compatibility and stereo performance result.
Furthermore, stereo modulation systems are disclosed herein which employ limiting circuits providing input signal limiting in such a manner that signal-to-noise characteristics are enhanced. The limiting circuits are designed to allow each input signal to exceed the amplitude which would produce 50% modulation of the transmitting signal, as long as total modulation constraints are not exceeded.
In accordance with the present invention, an AM stereo modulation system is provided which comprises a source of a carrier signal, and a stereo modulator which modulates the carrier signal in accordance with first and second source signals to provide a composite modulated signal having two phase components, the amplitudes of each of which are modulated in accordance with a corresponding one of the source signals, with the phase components being phased at equal phase angles on either side of the carrier signal by an angle of no greater than 20.degree. and no less than 10.degree..
In accordance with another aspect of the present invention, a limiting circuit is provided for applying amplitude constraints to the first and second source signals so as to supply the stereo modulator with amplitude limited source signals. The limiting circuit includes an individual input channel limiter for preventing the amplitude of each of the source signals from exceeding a selected level corresponding to at least 60% but no greater than 90% of the amplitude necessary to provide 100% modulation of the carrier signal, and a total modulation of the carrier signal from exceeding selected total modulation constraints.
Also in accordance with the present invention, an AM stereo demodulator is disclosed for demodulating the composite modulated signal. A quadrature demodulator responds to the composite modulated signal to recover in-phase and quad-phase components therefrom. The relative amplitudes of the recovered components are corrected in accordance with the tangent of the phase angle .theta. by which each of the two phase components carrying the source signals is shifted with respect to the carrier. A matrix circuit adds and subtracts the amplitude adjusted signals from one another so as to thereby recover the two source signals.